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Igneous Rocks
Igneous Rock Textures
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Tell the three types of igneous rocks apart by their texture, starting with the size of the mineral grains. Extrusive
rocks cool quickly (over periods of seconds to months) and have invisible or very small grains, or an aphanitic texture.
Intrusive rocks cool more slowly (over thousands of years) and have small to medium-sized grains. Plutonic rocks cool
over millions of years, deep underground, and can have grains as large as pebbles — even a meter across. Both intrusive
and plutonic rocks have phaneritic texture. Because they solidified from a fluid state, igneous rocks tend to have an
equigranular texture, a uniform fabric without layers, and the mineral grains are packed together tightly. Think of the
texture of a piece of bread as a similar example.
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In many igneous rocks, large mineral crystals "float" in a fine-grained groundmass. The large grains are called
phenocrysts, and a rock with phenocrysts is called a porphyry; that is, it has a porphyritic texture. Phenocrysts are
minerals that solidified earlier than the rest of the rock, and they are important clues to the rock's history.
Some extrusive rocks have distinctive textures. Obsidian, formed when lava hardens quickly, has a glassy texture. Pumice
and scoria are volcanic froth, puffed up by millions of gas bubbles giving them a vesicular texture. Tuff is a rock made
entirely of volcanic ash, fallen from the air or avalanched down a volcano's sides. It has a pyroclastic texture. And pillow
lava is a lumpy formation created by extruding lava underwater.
Igneous Rock Types: Basalt, Granite and More
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Igneous rocks are classified by the minerals they contain. The main minerals in igneous rocks are hard, primary
ones: feldspar, quartz, amphiboles and pyroxenes (together called "dark minerals" by geologists), and olivine along with
the softer mineral mica.
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The two best-known igneous rock types are basalt and granite, which differ in composition. Basalt is the dark,
fine-grained stuff of many lava flows and magma intrusions. Its dark minerals are rich in magnesium (Mg) and iron (Fe),
hence basalt is called a mafic rock. So basalt is mafic and either extrusive or intrusive. Granite is the light, coarse-grained
rock formed at depth and exposed after deep erosion. It is rich in feldspar and quartz (silica) and hence is called a felsic
rock. So granite is felsic and plutonic.
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These two categories cover the great majority of igneous rocks. Ordinary people, even ordinary geologists, use
the names freely. (Stone dealers call any plutonic rock at all "granite.") But igneous petrologists use many more names.
They generally talk about basaltic and granitic or granitoid rocks among themselves and out in the field, because it takes
lab work to determine an exact rock type according to the official classifications. True granite and true basalt are narrow
subsets of these categories. (Get deeper into classification)
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But a few of the less common igneous rock types can be recognized by non-specialists. For instance a darkcolored plutonic mafic rock, the deep version of basalt, is called gabbro. A light-colored intrusive or extrusive felsic rock,
the shallow version of granite, is called felsite or rhyolite. And there is a suite of ultramafic rocks with even more dark
minerals and even less silica than basalt. Peridotite is the foremost of those.
Where Igneous Rocks Are Found
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The deep sea floor (the oceanic crust) is made of basaltic rocks, with ultramafic rocks underneath. Basalts are
also erupted above the Earth's great subduction zones, either in volcanic island arcs or along the edges of continents.
However, continental magmas tend to be less basaltic and more granitic. (more on arc volcanism)
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The continents are the exclusive home of granitic rocks. Nearly everywhere on the continents, no matter what
rocks are on the surface, you can drill down and reach granitoid eventually. In general, granitic rocks are less dense than
basaltic rocks, and thus the continents actually float higher than the oceanic crust on top of the ultramafic rocks of the
Earth's mantle. The behavior and histories of granitic rock bodies are among geology's deepest and most intricate
mysteries.
Basalt, Scoria and Gabbro
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Basalt (pronounced /bəˈsɔːlt/, /ˈbæsɔːlt/, or /ˈbeɪsɔːlt/) is a common extrusive volcanic rock. It is usually grey to
black and fine-grained due to rapid cooling of lava at the surface of a planet. It may be porphyritic containing larger
crystals in a fine matrix, or vesicular, or frothy scoria. Unweathered basalt is black or grey. On Earth, most basalt magmas
have formed by decompression melting of the mantle. Basalt has also formed on Earth's Moon, Mars, Venus, and even on
the asteroid Vesta. Source rocks for the partial melts probably include both peridotite and pyroxenite (e.g., Sobolev et al.,
2007). The crustal portions of oceanic tectonic plates are composed predominantly of basalt, produced from upwelling
mantle below ocean ridges. The term basalt is at times applied to shallow intrusive rocks with a composition typical of
basalt, but rocks of this composition with a phaneritic (coarse) groundmass are generally referred to as diabase (also
called dolerite) or gabbro.
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Basalt is a dark-colored, fine-grained, igneous rock composed mainly of plagioclase and pyroxene minerals. It
most commonly forms as an extrusive rock, such as a lava flow, but can also form in small intrusive bodies, such as an
igneous dike or a thin sill. It has a composition similar to gabbro. The difference between basalt and gabbro is that basalt
is a fine-grained rock while gabbro is a coarse-grained rock. Basalt underlies more of Earth's surface than any other rock
type. Most areas within Earth's ocean basins are underlain by basalt. Although basalt is much less common on continents,
lava flows and flood basalts underlie several percent of Earth's land surface. Basalt is a very important rock..
Granite and Rhyolite
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Granite (pronounced /ˈɡrænɨt/) is a common and widely occurring type of intrusive, felsic, igneous rock. Granites
usually have a medium- to coarse-grained texture. Occasionally some individual crystals (phenocrysts) are larger than the
groundmass, in which case the texture is known as porphyritic. A granitic rock with a porphyritic texture is sometimes
known as a porphyry. Granites can be pink to gray in color, depending on their chemistry and mineralogy. By definition,
granite has a color index (the percentage of the rock made up of dark minerals) of less than 25%. Outcrops of granite tend
to form domes and rounded massifs.
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Granite consists of quartz (gray), plagioclase feldspar (white) and alkali feldspar (beige) plus dark minerals like
biotite and hornblende.
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Rhyolite is a high-silica lava that is chemically the same as granite but is extrusive rather than plutonic. Rhyolite
lava is too stiff and viscous to grow crystals except for isolated phenocrysts. The presence of phenocrysts means that
rhyolite has a porphyritic texture. Rhyolite is typically dark and has a glassy groundmass, but it may be white, pink or
reddish. Being high in silica, rhyolite is a stiff lava that tends to have a banded appearance. Indeed, "rhyolite" means "flow
stone" in scientific Greek.
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Rhyolite is typically found in continental settings where magmas have incorporated granitic rocks from the crust
as they rise from the mantle. It tends to make lava domes when it erupts. See 4.21c.
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Pegmatite is a plutonic rock with exceptionally large crystals. It forms at a late stage in the solidification of granite
bodies. Pegmatite bodies are thought to form predominantly in granites during their final stage of solidification. The final
fraction of mineral material is high in water and often also in elements such as fluorine or lithium. This fluid is forced to the
edge of the granite pluton and forms thick veins or pods. The fluid apparently solidifies rapidly at relatively high
temperatures, under conditions that favor a few very large crystals rather than many small ones. Pegmatites are sought
out by mineral collectors and gemstone miners not only for their large crystals, but for their examples of rare minerals. A
pegmatites near Antelope Lake features large rose quartz, books of biotite and blocks of K-feldspar.
Obsidian and Pumice
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Obsidian is an extrusive rock: lava that cooled without forming crystals, giving it a glassy texture. Pumice is
basically lava froth, an extrusive rock frozen as its dissolved gases come out of solution. It looks solid but often floats on
water. Pumice is much like scoria in that both are frothy, lightweight volcanic rocks, but the bubbles in pumice are small
and regular and its composition is more felsic than scoria's. Also, pumice is generally glassy whereas scoria is a more
typical lava with microscopic crystals.
Diorite and Andesite
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Diorite is a plutonic rock that is something between a granite and a gabbro. It consists mostly of white plagioclase
feldspar and black hornblende. Unlike granite, diorite has no or very little quartz or alkali feldspar; unlike gabbro it is fairly
light colored—half black and half white—and under the microscope it has sodic, not calcic plagioclase. If a dioritic rock is
erupted from a volcano (that is, if it is extrusive), it cools into andesite lava.
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The official definition of diorite is narrower than what the average geologist calls diorite. With more quartz, diorite
becomes tonalite. With more alkali feldspar, diorite becomes monzonite. With more of both minerals, diorite becomes
granodiorite.
Peridotite
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Peridotite makes up a large part of the Earth, but it's very seldom seen because it lies in the mantle, underneath
the crust. It took a rare combination of tectonic and geologic activity to get this piece of deep lithosphere across the ocean
floor, plaster it against the continent, carry it down into a subduction zone and back up again, then gently release it to the
rough hands of the Pacific surf, which are quickly battering it into sand to be altered to clay. Peridotite is a mediumgrained, dark-colored, intrusive igneous rock that contains at least 10 percent olivine, other iron- and magnesium-rich
minerals (generally pyroxenes), and not more than 10 percent feldspar.
Igneous Rock Kit from Wardʼs Natural Science contains the following: andesite, basalt, diorite, gabbro, granite,
obsidian, pegmatite, periotite, pumice, rhyolite (2), scoria. You must determine which numbered specimens are these
rocks. Work as a group with the people at your table. Each group will hand in a specimen list of the names of the
numbered specimens. Be sure everyone working in the group today has their name on the specimen list.
Igneous Rock Worksheet
Sample
1
2
3
4
5
6
7
8
9
10
11
12
Texture
(fine/course)
Main Minerals (if visible)
Light/
Intermediate/Dark
Intrusive/
Extrusive
Name